Since X-ray attenuates depending on an atomic number of an element configuring a material and a density and a thickness of the material, it is used as a probe for seeing through an inside of a subject. An imaging using the X-ray is widely spread in fields of medical diagnosis, nondestructive inspection and the like.
In a conventional X-ray imaging system, a subject is arranged between an X-ray source that irradiates the X-ray and an X-ray image detector that detects an X-ray image, and a transmission image of the subject is captured. In this case, the X-ray irradiated from the X-ray source toward the X-ray image detector is subject to the quantity attenuation (absorption) depending on differences of the material properties (for example, atomic numbers, densities and thickness) existing on a path to the X-ray image detector and is then incident onto the X-ray image detector. As a result, an X-ray transmission image of the subject is detected and captured by the X-ray image detector to obtain an image (hereinafter, referred to as an absorption image) based on an intensity change of the X-ray by the subject. As the X-ray image detector, a flat panel detector (FPD) that uses a semiconductor circuit is widely used, in addition to a combination of an X-ray intensifying screen and a film and a photostimulable phosphor (accumulative phosphor).
However, the smaller the atomic number of the element configuring material, the X-ray absorption ability is reduced. Accordingly, for the soft biological tissue or soft material, a difference of the X-ray absorption abilities is small and thus it is not possible to acquire an enough contrast of an image. For example, the cartilaginous part and joint fluid configuring an articulation of the body are mostly comprised of water. Thus, since a difference of the X-ray absorption amounts thereof is small, it is difficult to obtain the contrast of an image.
Regarding the above problems, instead of the intensity change of the X-ray by the subject, a research on an X-ray phase imaging of obtaining an image (hereinafter, referred to as a phase contrast image) based on a phase change (refraction angle change) of the X-ray by the subject has been actively carried out in recent years. In general, it has been known that when the X-ray is incident onto an object, the phase of the X-ray, rather than the intensity of the X-ray, shows the higher interaction. Accordingly, in the X-ray phase imaging of using the phase difference, it is possible to obtain a high contrast image even for a weak absorption material having a low X-ray absorption ability. As the X-ray phase imaging, an X-ray imaging system has been recently suggested which uses an X-ray Talbot interferometer having two transmission diffraction gratings (phase type grating and absorption type grating) and an X-ray image detector (for example, refer to Patent Literature 1).
The X-ray Talbot interferometer includes a first diffraction grating (phase type grating or absorption type grating) that is arranged at a rear side of a subject, a second diffraction grating (absorption type grating) that is arranged downstream at a specific distance (Talbot interference distance) determined by a grating pitch of the first diffraction grating and an X-ray wavelength, and an X-ray image detector that is arranged at a rear side of the second diffraction grating. The Talbot interference distance is a distance in which the X-ray having passed through the first diffraction grating forms a self-image by the Talbot interference effect. The self-image is modulated by the interaction (phase change) of the subject, which is arranged between the X-ray source and the first diffraction grating, and the X-ray.
In the X-ray Talbot interferometer, moiré fringes generated by superposition of the self-image of the first diffraction grating and the second diffraction grating are detected, and the phase information of the subject is acquired by analyzing a change of the moiré fringes by the subject. As an example of the method of analyzing moiré fringes, a fringe scanning method is proposed. According to the fringe scanning method, a plurality of imaging is performed while the second diffraction grating is translation-moved with respect to the first diffraction grating in a direction, which is substantially parallel with a plane of the first diffraction grating and is substantially perpendicular to a grating direction (strip band direction) of the first diffraction grating, with a scanning pitch that is obtained by equally partitioning the grating pitch. Then, an angle distribution (differential image of a phase shift) of the X-ray refracted at the subject is acquired from changes of signal values of respective pixels obtained in the X-ray image detector. Based on the acquired angle distribution, it is possible to obtain a phase contrast image of the subject.